The treatment of abnormally and physiologically dangerous high cardiac rate conditions, i.e., tachycardia and fibrillation, has become very important, and is the subject of considerable development. For example, modern implantable cardiac pacemakers can be provided with an anti-tachycardia feature, whereby the implantable device senses the occurrence of tachycardia, and responds by delivering one or more stimuli in a pattern designed to eliminate the dangerous high rate. Likewise, implantable defibrillators are designed to determine the occurrence of fibrillation, and to respond automatically with one or more defibrillation pulses. In this specification, the reference to abnormally high rates, or high rate conditions, incorporates the range of abnormal conditions, through all tachycardia rates up to and including fibrillation. Thus, the term tachycardia as used herein embraces the range of abnormally high rates, up to and including fibrillation. The treatment of such conditions, i.e., anti-tachycardia treatment or defibrillation, is referred to generally as cardioversion. The devices which can provide such cardioversion treatment can be specifically designed for a given treatment, i.e., defibrillators, or can be devices which combine pacing functions and one or more types of cardioversion treatment.
When implanting such an implantable device which provides for cardioversion, it is necessary to test the device operationally, and set the stimulus parameters so as to ensure reliable treatment. Thus, for a pacemaker having a feature of terminating ventricular tachycardia (VT), it is important to test at time of implant to determine that the prescribed pattern of anti-tachycardia stimulus pulses is delivered with the optimal parameters to control and terminate the VT for the patient receiving the implant. In order to carry out such a test, it is necessary to first induce the VT (or other high rate condition), and then set the anti-tachycardia stimulus parameters, e.g., number of pulses, timing and energy, for optimal termination of the VT. The aim of this invention is to provide the device with an improved and more reliable capability of inducing such a high rate abnormal condition, for test purposes.
It is known that tachycardia or fibrillation can be stopped, or terminated by delivery of stimulus pulses which interfere with the re-entry loop mechanism. Thus, in terminating such a non-physiological condition, a stimulus pulse is delivered shortly after the cardiac refractory period, and before the next expected spontaneous beat. In U.S. Pat. No. 4,390,021, there is disclosed the standard technique of delivering a pair of pulses, each timed with respect to the preceding QRS so as to fall within a "region of susceptibility," in order to break up the tachycardia. The system of the disclosure relies on searching to find the timing which results in break up of tachycardia, i.e., to find the "region of susceptibility." It is mentioned that a similar technique can be used to induce tachycardia, but no specifics are mentioned with regard to the required timing. It is noted that the timing for inducing tachycardia is not necessarily the same, relative to the QRS, as for breaking up tachycardia.
U.S. Pat. No. 4,593,695 discloses a successful technique for terminating VT which involves overdriving the ventricle with an early stimulus pulse, detecting the occurrence of the repolarization which is manifested by the T-wave, and then delivering a stimulus which is intended to break up the tachycardia. Since the T-wave is produced by cardiac cell repolarization and thus roughly indicates the end of the refractory period, it is necessary to wait until the T-wave detection in order to deliver a stimulus which can have an effect. Successive such early pulses can be delivered, as necessary to terminate the tachycardia condition. However, this reference gives no guidance concerning inducing tachycardia.
U.S. Pat. No. 5,129,392 is specifically directed to a technique of inducing fibrillation by delivery of a high voltage pulse following the refractory period of the heart chamber. The refractory period is disclosed to be determined by getting an average of the stimulus-T interval and delivering the high voltage pulse at a time based on this average and referenced to a preceding pacing pulse or spontaneous QRS. However, this technique has only about a 60% success rate, which means that often the delivery of a high energy pulse must be repeated. This produces substantial patient discomfort, and causes battery drain and consequent reduction of device longevity. Further, it is our observation that the timing with respect to the T-wave is critical, and accordingly it is desirable to do more than simply deliver an inducing pulse after the some predetermined interval following the QRS or the T-wave.
For an implantable device having an anti-tachycardia feature, it is desirable to be able to induce tachycardia using the vastly lower energy pacing pulses. This is the case whether the device is a pacemaker, or whether it incorporates cardioverting capacity, as with a combined pacemaker-cardioverter-defibrillator ("PCD"). In any case, it is highly desirable to provide the capability of inducing tachycardia by using only pacing-level pulses, to optimize patient comfort and save energy. This means that the tachy-including pulses are delivered as a series of smaller energy pulses. Since QT interval varies with rate, the delivery of the first inducing pulse likely has an effect on the following QT interval. The result of delivering a series of pulses is that each cycle the refractory interval may be slightly different, such that use of a fixed interval following the pacing stimulus or sensed QRS is relatively unlikely to be efficient in finding the proper time to induce VT. Accordingly, it is important to be able to determine on a cycle-to-cycle basis the best time to deliver the inducing pulse (or pulse pair). The solution provided by this invention is to look for the occurrence of a T-wave following a QRS which preferably is evoked by a pacing pulse; to determined the T-wave by near field sensing at the location where the pacing pulses are delivered; and to deliver one or more inducing pulses in closely timed relation to a specific portion of the sensed T-wave.